The present invention relates to a tungsten-containing maraging steel having high strength and high toughness.
Conventional maraging steels contain 18% nickel, 8% cobalt, 4% molybdenum, 0.4Ti and 0.1% aluminum. As the content of titanium in such maraging steels changes from 0.2% to 0.7%, the yield strength varies from 1400 Mpa to 2100 Mpa. From the 1970s, the supply of cobalt, strategic element became insufficient, thereby causing cobalt to be expensive. As a result, it is needed to provide a maraging steel of new composition.
The story of the development of, the applied field, and the advantages of maraging steel are summarily disclosed in an introduction "Notes on the Development of Maraging Steels" of "Source Book on Maraging Steels" edited by R. F. Decker and published by ASM, in 1979. This reference taught that maraging steel is a special alloy exhibiting high strength and high toughness and being used in the aeronautic industry, the precision machinery industry, the precision mold industry, and the defense industry. As the alloy is air-cooled after the solid solution treatment, a matrix structure of massive martensite having high toughness can be obtained. After the aging, substitutional type alloying elements, that is, nickel, molybdenum, tungsten,, and titanum, which are supersaturated in the matrix structure of the alloy, forms such precipitations as Ni.sub.3 Mo, Ni.sub.3 W, and Ni.sub.3 Ti. These precipitations contribute to the achievement of high strength. On the other hand, high toughness can be obtained, as carbon in the alloy is maintained in an amount no more than 0.03%.
S. Floreen, etc., Trans. ASM, 57, 714-726(1964) taught that cobalt acts to reduce the solubility of the substitutional type alloying elements (nickel, molybdenum, and titanium) in the matrix structure of the alloy, and thus promotes the precipitation of Ni.sub.3 Mo. However, they also set force that cobalt by itself is not the constitution element of the precipitations. On the other hand, tungsten exhibits the physical property similar to that of molybdenum, in view of the fact that there is no difference of the valence electrons between both elements and that the lattice constant influencing the lattice deformation is 3.15 .ANG. in molybdenum and 3.17 .ANG. in tungsten.
From the above-mentioned two facts, the inventors found that it is possible to develop a new maraging steel wherein cobalt is eliminated and molybdenum is substituted by tungsten. The depreciation in the strength due to the elimination of cobalt could be compensated by increasing the content of titanum to 1.4%.